The short answer for peppered corydoras
Keep six peppered corydoras as the minimum group, and eight to ten if your tank footprint and filtration comfortably support them. Peppered corydoras are shoaling catfish from South American rivers, where they move over sandy bottoms in company. That social wiring does not switch off in a home aquarium. A lone cory or a pair will often spend the day tucked behind wood or plants, emerging nervously, while a full shoal spreads out across the substrate, barbels working through the sand in a relaxed, busy row.
Numbers matter here more than tankmates of other species. A peppered cory surrounded by tetras but no other corydoras is still, from its perspective, alone. The companionship it recognises is its own kind.
What a too-small group looks like
Under-grouped peppered corydoras give clear signals once you know them. They freeze for long stretches instead of foraging. They startle hard at movement near the glass and dart into cover. Their colour looks pale compared with the peppered patterning a settled fish shows, and they may refuse food while bolder tankmates feed above them. None of this means the fish is ill; it usually means the shoal is too small for the fish to feel the safety that numbers provide in the wild.
The fix is almost always more of their own species, added thoughtfully, rather than new decorations or a different food. Confidence in this species is a group property, not an individual trait.
Making room for a proper shoal
Plan the group around bottom space, not just water volume. Peppered corydoras live on the floor of the tank, so a longer, wider footprint serves six fish better than a tall, narrow column of the same capacity. Choose soft sand or smooth, fine substrate: their barbels are delicate, and rough or sharp gravel can wear and damage them as they dig for food. Cover matters too. Pieces of wood, smooth stones, and plant clumps give the shoal resting shade between foraging bouts, which paradoxically makes them more visible, because fish with a retreat nearby venture out more.
Stock the group into a cycled, stable tank, and add them together where you can. Six fish arriving at once settle as a unit; adding one fish at a time repeatedly puts a single nervous newcomer in front of established tankmates.
Feeding and keeping the group thriving
Feed peppered corydoras at their level. Flake drifting down is unreliable for bottom fish in a community tank, so offer sinking foods that reach the substrate, ideally after the main lights dim, when corydoras naturally become active. Watch the whole shoal at feeding time: in a healthy group, every fish should be out nosing through the sand. A fish that consistently stays behind cover while the others eat deserves a closer look and, if it is losing condition or its barbels look eroded, a conversation with an aquatic veterinarian or experienced fish health adviser.
Keep the water clean and steady rather than chasing perfect numbers. Regular partial water changes, a varied diet, and stable temperature suit this hardy but shoal-dependent species far better than constant adjustment.
A group everyone in the house understands
Because the right number is a commitment — six fish, bought together, fed at the bottom, counted at headcounts — it helps when the whole household works from the same notes rather than one person's memory. On KinsLe, Family Sharing lets the people who help with your peppered corydoras view the same profile and care details, so the shoal size, feeding routine, and substrate notes stay consistent whoever is looking after the tank.